In communication networks, optical communication using optical fibers as transmission path segments is often used. To cope with a recent increase in the amount of information streaming through a network, increasing the transmission distance and transmission capacity of an optical transmission system is required. However, the transmission distance and transmission capacity of the optical transmission system are limited due to optical waveform distortions in optical fibers. The optical waveform distortions include linear distortion and nonlinear distortion.
The linear distortion includes, for example, chromatic dispersion and polarization mode dispersion. The linear distortion can be compensated for using a digital coherent technique and an equalizing technique based on digital signal processing, or using an optical compensating technique in an optical receiving device receiving an optical signal transmitted through optical fibers. The nonlinear distortion includes, for example, cross-phase modulation and self-phase modulation. Compensation for the nonlinear distortion in an optical receiving device is disclosed in, for example, Kazuro Kikuchi, “Electronic Post-compensation for Nonlinear Phase Fluctuations in a 1000-km 20-Gbit/s Optical Quadrature Phase-shift Keying Transmission System Using the Digital Coherent Receiver”, OPTICS EXPRESS, Vol. 16, No. 2, pp. 889-896, 2008.
According to the disclosed technique, nonlinear distortion caused in optical fibers can be compensated for in an optical receiving device. However, the above-described technique does not detect nonlinear distortion. Accordingly, the accuracy with which to compensate for nonlinear distortion is limited.